1. Field of the Invention
The present invention generally relates to grate coolers for the heat treatment of particularly fine bulk material transported along such grate coolers, and in particular to the support or the suspension of a mobile framework of such grate coolers as well as to the interaction of its mechanical wear and the process efficiency of the grate cooler.
2. Prior Art
Conventional grate coolers comprise an upper housing and a lower housing, the latter being pressurised by a cooling medium for the bulk material; a plurality of transverse rows of grate plate (with respect to the direction of transport of the material), wherein the rows are at least partially mobile, and the grate plates partially overlap each other; a mobile framework consisting of transverse beams and longitudinal beams carrying the transverse mobile rows of grate plates; and either one of a support of the mobile framework comprising axles running on rollers or rotating axles with annular rails running on flat rails, or suspension means for the said mobile framework formed by long elongated tension strands, replacing the aforementioned axles or rollers, etc.
In order to effect the material transport the mobile framework is stroked back and forth. Normal operation is carried out at between 3 to 30 strokes per minute of 70 to 150 mm length as allowed by the overlapped length of consecutive rows of grate plates. The strokes are generated either mechanically by excenter drives or by hydraulic drives.
The majority of hitherto utilised grate coolers comprise a mobile frame or framework, which is supported either by axles running on rollers, or by rotating axles with annular rails running on flat rails. The lateral guidance of the mobile frame is provided by flanges of the rollers or rails. In many cases there exist additional thrust rollers. The frame may be stiffened further by diagonal tension rods. Due to the wear of rollers, roller bearings, and rails the mobile frame gradually works its way down thereby losing its resistance property. About once a year the supported mobile framework has to be reset and/or realigned.
In order to overcome such regular maintenance and repair requirements there has been developed a structure in which the framework is suspended by long tension strands arranged in parallel to the perpendicular, longitudinal plane of the grate cooler, and inside of compartments attached to the upper housing portion of the grate cooler. The lateral guidance is provided by friction plates attached to the transverse beams, which friction plates engage counter plates fixed to the lower housing portion. In addition, tension chains extend for the longest distance between the mobile frame and the undergrate housing. Despite its merit with respect to the wear of rollers and rails, this design has finally been abandoned because of lack of lateral stability of the mobile frame.
Extra rigidity of the mobile framework is required in the case of a hydraulic grate drive with opposite pairs of hydraulic cylinders. The work loads of the cylinders may become so uneven that the cylinder of one side performs practically all the work while the opposite one runs idle. The rigidity of the mobile frame has to compensate for such uneven work loads.
Consequently, the flanged rollers, thrust rollers, or friction plates, or whatever may serve as a mechanical lateral guide of the mobile frame, are subject to wear. The wear adds to the allowances made for the thermal expansion of the rows of grate plates and results in a fairly large lateral clearance and consequently, in a decrease in the overall resistance of the grate.
This grate resistance has been an essential problem in grate cooler development and design over the past years. It is a main aim to design grate coolers of optimal resistance against the passage of the cooling medium. According to recent improvements the quality of grate coolers depends largely on their resistance against the passage of the cooling medium as the resistance warrants the even distribution of the cooling medium into the bulk material (cf. K. von Wedel and R. Wagner) "Are cooling grates clinker coolers or heat recuperators? Theoretical and practical limits of cross-flow cooling. Translated from the journal "Zement-Kalk-Gips", 37th volume, 5/1984, page 244-247). Said resistance is achieved by fine and evenly distributed openings of the grate surface. With this in mind, a grate has been developed (U.S. Pat. No. 4 600 380) comprising grate elements in the form of a box to be placed upon individually aerated, hollow grate beams. Said hollow beams and grate elements, taken per se, show the desired resistance property. However, this does not influence the aforementioned losses in the lateral guide regions, as the narrow openings of said hollow beams constitute the resistance of the major part of the grate surface, only. As long as there are uncontrolled openings or openings subject to wear, the benefits of the resistance concept cannot be fully utilized. The uncontrolled openings are like weak links in a chain.
As a consequence of said loss of grate resistance the hazard of channelling of the cooling medium grows. Channelling is observed especially at the sides, which also causes wear of the refractory lining by sandblasting. The grate's life between repair campaigns is often limited by the wear starting from one of the two lateral clearances. Once the guiding flanges or friction plates no longer limit the course of the mobile frame, the friction between mobile grate plates and side boards takes over. As wear proceeds at one side, clearance at the opposite side becomes larger.
3. Objects of the Invention
It is the main object of the present invention to suggest a grate cooler whose lateral clearances can be designed narrow enough to meet the requirements for a high resistance grate cooler.
It is another major object of the invention to provide a practically wearless lateral guide of the mobile framework.
It is yet another essential object of the present invention, particularly with regard to said known grate coolers incorporating hollow beams and box-shaped grate plates arranged on the beams, to match the lateral clearances between the mobile beams and the fixed structure of the grate cooler as well as the clearances between fixed and mobile rows of grate plates to the resistance achieved by said hollow beams.